Q&A: Carpet Filtration Mystery

Firewall Details

Q.We are builders in
the Philadelphia area. Over the past few years,
we’ve experienced several cases of what
we’ve been told is "carpet filtration"
— dark ghost lines that appear on the carpet
under doors and around the perimeter of rooms. We
tested the hvac system and found a sizeable imbalance
between the supply and return sides, so we added an
additional central return system to equalize the
pressures. Is the imbalance the major cause of the
filtration? If not, what are the causes? We have had to
replace some carpet, and hope that we don’t
continue to be liable for a recurring
problem.

A.Frank Vigil, of
Advanced Energy in Raleigh, N.C., responds:
You are correct in assuming that a pressure
imbalance is causing the carpet to filter
particulates out of the air. However, simply adding
additional return air is not likely to correct the
situation.

In dealing with carpet filtration, the most
effective strategy is to remove the source of the
soiling. A number of things can cause carpet
stains, from simple dirt and dust to more complex
sources such as soot from candles or improperly
functioning combustion appliances. Get rid of the
source and you’ll get rid of the
problem.

Second, try to determine the driving force
acting upon the pollutant that is causing the
filtration. Three basic rules apply. First, in
order to have air flow, you must have a hole and a
driving force. Eliminate either of these, and there
will be no air flow. Second, for every cubic foot
of air that leaks into a house, one cubic foot must
also leak out. Thus, any air entering a home
requires that air is also exiting. Third, high
pressure always moves to low pressure.

With this in mind, you can understand the three
driving forces that create pressure imbalances:
heat (stack effect), wind, and fans. These can work
independently or together to cause negative or
positive pressures inside the building.

Stack effect depends on two things: the
difference in temperature between the inside and
outside of the building, and the height of the
building. The greater either of these are, the
greater the stack effect.

Wind can enter a building through cracks,
crevices, and holes around the framing, windows,
and doors. Again, as the wind leaks in, air must
leak out. Keep in mind that whatever is in the wind
leaking in (moisture, dirt, dust, pollen, and so
forth) will now also be in the house.

Last, and possibly most important, are the
effects of fans in the house. Three types of "fan
effects" are caused by leaky ducts, exhaust fans,
and the closing of interior doors. Where any
portion of a duct system is located outside the
heated space, leaks can cause pressure imbalances.
An airtight duct system would draw one unit of air
through the return from inside the house and send
that one unit — now conditioned —
back to the house. But if there are leaks on either
the return or the supply side, more air will either
be taken from the building or supplied back to it,
causing either negative or positive pressures.

Exhaust fans also create pressure imbalances,
because they exhaust air from inside the house to
the outside. Unless there is sufficient makeup air
available, the house can be under negative pressure
with respect to the outside.

The last fan effect is caused by closing
interior doors. In houses with only one or two
returns, closing the door to an interior room
restricts the free flow of air from that room on
its way back to the return air grille. This
"starves" the house for that air, causing the
portion of the house with the return grille to be
under negative pressure with reference to the
outside, and the bedroom to have positive pressure.
When the bedroom door is closed, the return grille
draws air through the gap at the threshold, causing
the carpet to filter out dirt or soot and leaving
the characteristic stain along the bottom of the
door.

Hire a qualified energy contractor to determine
the driving forces causing the filtration. Once you
know the problem, you can use proper building
techniques, many of which have been discussed in
previous issues of JLC, to eliminate it.
These include:

proper air sealing of the building shell to
minimize wind intrusion

sealing the upper half of the building (all
holes, bypasses, chases, and so forth, between
the house and attic) to minimize stack
effect